Age related chances in the immune system are being investigated. Specifically, the changes in susceptibility to the induction and termination of acquired immunological tolerance to human immunoglobulin is being examined in several strains of mice. The mechanisms of the increased incidence of autoimmunity which occurs with advancing age is being investigated using the murine thyroiditis model in which the autoimmune lesion is induced with cross-reacting heterologous thyroglobulins. These studies have shown that mice become more resistant to tolerance induction with age and that in spite of this more suppressor cell activity is found in old age. B cell tolerance is especially labile. There is a difference in susceptibility to tolerance induction between pairs of mouse strains congenic at the H-2 locus. This difference is seen in both young (2 month) and middle aged (12 months) animals.